1. Field of the Invention
This invention relates, generally, to polymer films. More particularly, it relates to thermally stable piezoelectric polymer foams and method of fabrication thereof.
2. Description of the Prior Art
Piezoelectrets or ferroelectrets are space-charged porous polymers with significant piezoelectricity. [1,6,7,93] The cellular voids with charges of opposite signs on the upper and lower walls form macroscopic dipoles. The effective dipole moment varies under mechanical stress, which gives rise to the piezoelectricity. [5,32,60] Compared with traditional inorganic piezoelectric materials, polymer ferroelectrets show attractive characteristics such as flexibility, light weight, high piezoelectric coefficient d33 and low cost. To date, the most intensively studied ferroelectrets are based on the cellular polypropylene (PP) films, which exhibit high piezoelectric coefficients of several hundred to over one thousand pC/N. However, their low operation temperature (normally lower than 60° C.), which results from the poor charge-storage stability of PP at elevated temperatures, [98] restricts their application. Therefore, various strategies have been explored to improve the thermal stability of the ferroelectrets. For example, Rychkov et al. [99] showed that by orthophosphoric acid treatment the piezoelectric coefficient decay curves of the ferroelectrets (polyethylene based) can be shifted to higher temperatures by 40 K. Nonetheless, the majority of the efforts in this area are development of ferroelectrets from more thermally stable polymers.
The optimization of the voids structure to reduce the elastic stiffness may be one critical factor influencing the piezoelectric properties of polymer ferroelectrets. [62,63] Normally an anisotropic void structure is required by polymer ferroelectrets for higher piezoelectric activity. [64] Two different fabrication techniques were proposed for generating this type of void structure. The most commonly investigated approach is based on the combination of biaxial stretching and high pressure treating techniques. However, this method has primarily been limited to the development of cellular polypropylene (PP) due to the difficulty in the creation of cellular structure. For most polymers, only structures with low porosity and thick compact layers can be prepared. An alternative, macroscopic approach has recently been found. Patterned polymer films were assembled to form void structure by bonding. The most common bonding methods used in ferroelectrets include thermo compression and laser ablation, focused on fluoropolymer systems. Although these bonding methods are easy and fast, their applications are restricted in the design of simple two-layer structure or sandwich structure.
In the design of ferroelectrets, another key factor is charge stability of ferroelectret materials. As an example, although the extensively-studied cellular PP ferroelectrets have exhibited very high piezoelectric coefficient of several hundred pC/N, the operating temperature of PP ferroelectrets is normally lower than 60° C. due to the low charge storage stability of PP at elevated temperature. Over fifteen years ago, it was reported that cyclo-olefin copolymers (COCs) exhibited excellent stability of positive surface charge, superior to any known positively charged polymer. [65,66] This, combined with low water adsorption and good mechanical properties, indicate that COCs are likely the most promising candidates for thermally stable ferroelectret material. However, their high elastic stiffness limits their piezoelectric activity to about 15 pC/N. [63,68] Thus the potential value of COC as a ferroelectret material may be diminished. Up to now, no suitable technique has been available for fabricating COC ferroelectrets. In contrast, the progress made in the fabrication process of other thermally stable ferroelectrets has recently been reported, which mainly focused on fluoropolymers.
Traditionally, the primary materials with reasonably large piezoelectric effects are single-crystalline materials such as ferroelectric relaxors (PMN-PT or PZN-PT) and polycrystalline ceramics such as lead zirconate titanate (PZT). Ferroelectrets are space-charge electrets [1-7] made from polymer foams that show strong piezoelectric activity, resulting from the macroscopic dipoles residing in the pores [1]. Their piezoelectric activities rival those of the best ceramic based materials [1,8]. A comparison of piezoelectric coefficients of several piezoelectric materials as seen in the prior art is presented in Table 1.
TABLE 1Comparison of piezoelectric coefficients of several piezoelectricmaterials.Piezoelectric materiald33 (pC/N)Crystal:2 (d11)Quartz (silicon dioxide)Ceramic:170-600Lead zirconate titanate (PZT)Ferroelectrics:20β-phase polyvinylidene (β-PVDF)Ferroelectret:600optimized cellular polypropylene(PP)
Currently the only commercially available ferroelectrets are based on porous polypropylene (PP) [8,9] films which have been applied in various devices, i.e., audio devices [10] as microphones [11], force sensors [12-14] and actuators [15], and respiration detectors [16]. They do not have sufficient thermal and UV stability and thus have limited uses. The low operating temperature (−20° C.˜50° C.) limits their usefulness. On the other hand, experimental ferroelectrets based on thermally stable polymers have poor piezoelectric activity largely due to lack of understanding of the processes to produce foams with engineered cellular morphology, and in-depth understanding of the processing-morphology-electro-mechanical response. This status quo has a historical origin that was initiated by physicists whose main interest is exploring the dielectric and piezoelectric properties using existing materials instead of development of new materials systems.
Conventional Fabrication of Ferroelectrets
Several steps are involved in the fabrication of ferroelectrets: generation of initial pores, post modification and charging.
Generation of Initial Pores:
The generation of the initial pore structure is typically done in two distinctive ways. The first approach, which is the current industrial practice of producing PP ferroelectrets [22], is a co-extrusion of a multilayer structure followed by biaxial stretching. The middle layer is a film of PP with micron sized particles. Upon stretching, micro-cracks form near the particle region due to higher stress level, resulting in lens-like anisotropic pores.
Recently, direct foaming using a blowing agent has received considerable attention [23-28]. In this approach, the bulk polymer film is placed in a high pressure chamber and saturated with a blowing agent such as carbon dioxide until equilibrium is reached. Upon pressure release or temperature increase, the gas oversaturation in the polymer leads to formation and growth of bubbles.
Post-Modification:
Following initial generation of the pores, the cell morphology (cell size and shape, and bulk density) can be further modified by stretching, inflation/expansion and stabilization.
Charging:
Charging of the porous polymers is typically done by corona charging. The mechanism is via dielectric barrier microplasma discharges (microstorms) within the pores. [29,30] When the porous materials are subjected to an electric field higher than a critical threshold, dielectric breakdown takes place in the air gap. The microdischarges generate many tiny microplasma discharges, resulting in a current flow and transfer of charges across the air gap. The charges generated are trapped in surface states and form oriented macroscopic dipoles responsible for the large piezoelectric effect. The current pulses are self-extinguished and nondestructive with negligible impact to the cell morphology.
The cell morphology has profound effect on the electromechanical properties of the ferroelectrets. For better charge storage capability and stability, closed cell foams are preferred, in which the pores (cells) are isolated with each other and are surrounded by complete cell walls. On the other hand, charges are more prone to drift and decay [31] in open cell foams where cell walls are broken or even absent and only ribs and struts are left.
Initial theoretical modeling based on a simplified layered structure [4] predicts d33∝∈σf(s1,s2)/Y where d33 is the piezoelectric coefficient (in the film thickness direction and aligned with the electrical field), ∈ is the dielectric constant of the matrix material, a is the stored electrical charge which is directly proportional to the total available surface area within the pores, f(s1,s2) is a function of the total thickness of solid layer (s1) and porous layer (s2) and therefore bulk density of the foams, and Y is the elastic modulus of the foams. Whereas the detailed structure—mechanical property relationship using finite element method [32,33] is only preliminary, a simple theoretical analysis [34] has revealed that the elastic modulus is directly proportional to the cell anisotropy (the ratio of the height to the diameter for lens-like cells) and the square of the density of the foams.
Two closely associated gaps exist that mitigate the advancement of ferroelectret materials, as described below.
Materials gap: The foremost challenge in current ferroelectrets is the lack of materials that possess high piezoelectric activity at elevated temperatures. PP ferroelectrets typically have a service temperature less than 60° C. While d33 remains stable over periods of years at room temperature, it decays with time constants of the order of days or minutes at 70° C. and 90° C., respectively [35]. It also has very low UV stability [36]. While chemical modification improved the thermal stability and enhanced piezoelectric activity [37], new materials or film systems with better thermal stability are being actively sought. Several groups of materials have been explored, such as cyclo-olefin copolymers (COC) [38-40], poly(ethylene terephthalate) (PET) [23-25], poly(ethylene naphthalate) (PEN) [26, 27], polycarbonate (PC) [71,72], and polyetherimide (PEI) [72,73]. Ferroelectrets from these materials exhibit higher stabilities than PP but possess lower d33 coefficients. High temperature engineering thermoplastics were also investigated [28] and improved charge storage stability was observed. For example, over 95 percent of charge is still retained after 1,400 min at 90° C. for poly(etherimide) (PEI) foams [28]. The foam has low porosity and non-uniform cell size distribution. No piezoelectric coefficient was reported.
COCs are also promising candidate materials for thermally stable ferroelectrets. They show excellent storage stability of positive surface charges (superior to any known positively charged polymer). [65,66] Their potential is further augmented by the low water adsorption, exceptional solvent and environmental stability, low dielectric constant and dielectric losses, and excellent mechanical and thermomechanical properties. [100] However, the progress on COCs ferroelectrets to date has been disappointing. There are only scant studies, achieving d33 about 15 pC/N. [38,39,67,70]
The other group of candidate materials is fluoropolymers (poly(tetrafluoroethylene) or PTFE, and its copolymers), which have already been widely used as polymer electrets due to their high temperature stability [41], exceptional chemical resistance, excellent insulating properties and charge storage capacity and stability, low dielectric constants and extremely small dielectric losses [42,43].
Despite their substantially higher thermal stability than PP, their low mechanical properties and severe creep behavior (continuous deformation under constant force) would constrain their use in many cases. Moreover, most fluoropolymer-based ferroelectrets contain large amount of open porosity, which is detrimental to the long-term electromechanical stability due to the potential charge de-trapping along the surface of the fibrils. [31,52]
Due to the intrinsic processing difficulties [44], open porous PTFE films produced by a complex process were used as ferroelectret materials [45-50]. To overcome the difficulty resulting from open porous morphology, layered sandwiched structures (a porous PTFE layer between two fluorinated ethylene-propylene copolymer (FEP) films) were explored. [31, 50-54] A value for d33 of ˜1500 pC/N has been reported for newly prepared samples [52] (note that this is on the order of the best single crystal relaxor ferroelectric materials), while stacks of multiple nonporous and porous film in alternating sequence [55, 56] lead to even higher d33 ˜2200 pC/N [56]. Sandwiched structures were also prepared using open porous amorphous Teflon (Teflon AF) with nonporous films. [57] Despite substantial improvements in thermal stability of these materials over PP, their long-term stability remains problematic due to charge de-trapping along the surface of the PTFE fibrils [31], which is accelerated by the matrix relaxation [51]. For charge storage and charge storage stability, closed cell morphology is intrinsically superior and preferred but is technically challenging to generate. This type of work is extremely scarce. Only one reference is available in the open literature for FEP ferroelectrets, in which a small d33 value (˜50 pC/N) was reported [58].
Knowledge/Technology gap: Many knowledge/technology gaps exist due to the highly interdisciplinary nature of the subject. In the context of basic materials processing research, the key gaps are i) lack of sufficient understanding of the impact of cell morphology and materials structure on the mechanical and electro-mechanical responses, which in turn results from: ii) lack of understanding of the materials and processes to produce the desired morphologies and structures; and iii) lack of development of such understanding by fundamental nonlinear electro-mechanical modeling.
The gaps described above can be primarily attributed to the fact that current practice of process parameter selection for foaming does not take into detailed consideration the material properties and is quite heuristic in nature. Other than the well-established process for making PP ferroelectrets, work on foaming of other polymers for ferroelectric applications is still in its infancy. Consequently current experimental thermally stable ferroelectrets being explored possess limited range of poor cell morphology.
One of the key challenges in developing smart materials for actuation and sensing is obtaining high energy density. Whereas conventional smart materials such as shape memory Nitinol, piezoelectric PZT, and magnetostrictive Terfenol-D have relatively large forces, trade-offs in weight penalties (solenoids, actuator density), heat transfer (Nitinol), and environmental issues (lead in PZT) motivate the need for light-weight alternatives with superior electro-mechanical efficiencies. This is a grand challenge for areas such as active flow control where minute texture changes on an aircraft wing can create dramatic changes in lift-to-drag ratios. Furthermore, position specific sensing and actuation capability of the ferroelectrets by rational architecture design [21,59] allow for distributed flow sensory actuation (collocated control) which is considerably more stable than separate sensor-actuator configurations. Light-weight skins using ferroelectrets are envisioned as a viable alternative to make this a reality. Moreover, many other applications include robotic actuators for legged robotics that require high agility in rescue missions or surveillance. Soft actuators are also more amenable to artificial limbs for grasping objects, precision surgical tools or artificial, self-powered organs that more closely match the modulus of biological tissue.
By addressing the materials and processing aspects in conjunction with fundamental understanding of the materials behavior through modeling, the morphology and properties of the ferroelectrets can be well engineered, leading to revolutionary soft piezoelectric materials with tailored mechanical and electro-mechanical response that can be integrated into microelectronics, energy conversion system, and various sensing and actuation systems.
The combining of two processes is proposed: laser cutting and CO2 bonding technology. By aligning the COC grids in a specific manner (detailed in various embodiments presented herein), large deformation in the thickness direction of COC ferroelectrets can be achieved. COC ferroelectrets with significant piezoelectricity may be prepared by applying foaming technology to achieve better manipulation of cell structure for enhanced electro-mechanical properties. However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill how the art could be advanced.
While certain aspects of conventional technologies have been discussed to facilitate disclosure of the invention, Applicants in no way disclaim these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein.
The present invention may address one or more of the problems and deficiencies of the prior art discussed above. However, it is contemplated that the invention may prove useful in addressing other problems and deficiencies in a number of technical areas. Therefore, the claimed invention should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein.
In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.